The invention relates to a fixing unit used in a printer such as an electrophotographic printer, for fixing images on a recording sheet.
In a printer such as an electrophotographic printer, a fixing unit is provided for fixing the toner image on the recording sheet. The fixing unit includes a heat roller accommodating a heater therein and a press roller having a surface made of an elastic member. A recording sheet is pressed and heated between the heat roller and the press roller, so that the toner image is fixed on the recording sheet.
Generally, the heat roller is rotated by a driving mechanism, while the press roller is supported freely. Further, the press roller is moved toward and away from the heat roller. That is, the press roller is moved between (1) an operating position where the press roller presses the recording sheet to the heat roller and (2) a retracted position where the press roller is separated from the heat roller.
The heat roller is rotated in synchronization with the feeding of the recording sheet (generally performed by a tractor unit). If the feeding of the recording sheet is stopped while the recording sheet is in contact with the heat roller for a long time, the recording sheet may dry out. In order to avoid this problem, the printer is so arranged that the recording sheet is held (pressed) by the heat roller and the press roller only when the recording sheet is being fed. That is, the press roller is moved to the operating position after a feeding of the recording sheet is started.
However, the press roller is not rotated when located at the retracted position, while the recording sheet is fed with a certain speed. Thus, there is a significant speed difference between the recording sheet and the press roller at a moment when the press roller contacts the recording sheet. If one lateral end of the recording sheet is brought into contact with the press roller earlier than the other lateral end of the press roller, the end which initially contacts the recording sheet may decelerate for a moment, while the other lateral end may decelerate. This causes a so-called skew wherein the recording sheet is inclined with respect to the feeding direction of the recording sheet.
Further, if the above-described deceleration of the recording sheet occurs at the fixing unit, it may decelerate the recording sheet passing through a transfer unit (located at an upstream of the fixing unit) where the toner image is transferred on the recording sheet. Thus, it may cause a so-called jitter wherein the transfer position of the toner image on the recording sheet is fluctuated.